Understanding reduced filament-motor systems is an important step
towards comprehending more complex active systems. Therefore
theoretical studies have investigated aspects of MT and motor-protein
assemblies at different scales
[1]}, [2]}, [3]}, [4]}, [5]}. Inspired by
the experiments of Sanchez et al. [6]}, Giomi et al. [7]}, [8]} and Thampi et al.
[9]}, [10]}, [11]} have studied liquid crystal
hydrodynamic models driven by an apolar active stress [12]}.
While apolar models reproduce qualitative features of these
experiments, MTs have polarity and crosslinking motors move
directionally; hence, aligned MTs must have different interactions
than anti-aligned MTs, and activity-driven material stresses and
fluxes should reflect the polarity of these interactions. We
investigate this through multiscale modeling, first discovering two
separate microscopic sources of active and extensile stresses, one
induced by motor driven polarity-sorting of anti-aligned MTs,
and another from relaxation of crosslink tethers between polar-aligned
MTs. We formulate a Doi-Onsager model
[13]}, [14]}, [15]}, [16]} with fluxes and stresses reflecting
these effects, and use this to study the Sanchez et al.
interfacial experiments. Simulations show persistent folding flows and
defect birth and annihilation, arising from active stresses occupying
geometrically distinct regions. Having properly accounted for drag of
the bounding fluids, we find a well-defined characteristic
length-scale from linear theory which agrees well with feature sizes
in our simulations.
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